Short-wavelength laser light sources have been commercially implemented in a wide variety of applications ranging from laser projectors to high-density optical storage devices. The short-wavelength laser light source outputs laser light in blue, green, or other color by using a wavelength conversion element which converts infrared light, i.e., light at the fundamental wavelength that a laser device as an optical device produces, into light at the second harmonic of that wavelength. The wavelength conversion element is formed using a crystal material such as LN (lithium niobate: LiNbO3) or LT (lithium tantalate: LiTaO3), but the harmonic wavelength conversion efficiency of such crystal material has temperature dependence, and hence the property that the conversion efficiency greatly varies due to variations in ambient temperature.
FIG. 26(a) is a graph showing one example of the variation of the harmonic output (HFO) of a wavelength conversion element as a function of the ambient temperature (T) of the wavelength conversion element. As can be seen from the graph shown in FIG. 26(a), the output of the wavelength conversion element drops in regions where the ambient temperature is low, and the output also drops in regions where the ambient temperature is high. Since the harmonic output of the wavelength conversion element greatly varies due to temperature variations as depicted, a temperature characteristic correcting means for correcting the temperature characteristics of the wavelength conversion element is indispensable in order to achieve good conversion efficiency and to obtain stable laser light at the harmonic wavelength. It is known to provide a laser light source having a wavelength conversion element mounted with a heater in order to adjust the temperature of the wavelength conversion element to a desired value (for example, refer to patent document 1).
FIG. 26(b) is a diagram showing a short-wavelength laser light source disclosed in patent document 1. As shown in FIG. 26(b), the short-wavelength laser light source is constructed by mounting a 0.8-μm semiconductor laser 410 and a wavelength conversion element 420 on a silicon substrate 401. A fundamental wave 412 is output from an active layer 411 in the semiconductor laser 410, and is introduced into an optical waveguide 421 formed within the wavelength conversion element 420 which outputs blue laser light 430 at the second harmonic wavelength. A groove 402 is formed by etching in a portion of the surface at which the silicon substrate 401 contacts the wavelength conversion element 420.
A thin-film heater 422 constructed from a Ti film is formed on the lower surface of the wavelength conversion element 420, that is, near the optical waveguide 421. By energizing this thin-film heater 422, the wavelength conversion element 420 can be maintained at the desired temperature. Further, since the presence of the groove 402 in the silicon substrate 401 serves to prevent the thin-film heater 422 from contacting the silicon substrate 401, the heat from the thin-film heater 422 is not easily conducted to the silicon substrate 401.
In a semiconductor laser device having temperature-dependent output characteristics, it is also known to provide a strip-like heater near the optical waveguide in order to reduce power consumption (for example, refer to patent document 2).